This is really useful. I've had a major frustration this week when I had the first 3 chords of a progression down, could 'hear' the fourth chord in my head, but couldn't find it on the keyboard. I'm going to see if what I'm looking for is a common progression.

Yes. Quite useful. This is basically just the "three-pie-slices-chart chords" of the good old circle of fifths (don't know the english word for it) - where you take the three majors (tonic, dominant and subdominant) and its relative minors.

For C that will be G and F, and relative Dm, Am, Em. Then you can rotate these 3/6 chords any way you want: